Fuel cells are electrochemical devices that convert the chemical energy in fuel directly into electricity. Unlike many devices for generating electricity, fuel cells do not require combustion. This enables fuel cells to operate at relatively high efficiencies and to produce relatively low emissions, making them an attractive source of electrical energy.
FIG. 1 illustrates a conventional fuel cell power generator 5. The fuel cell power generator 5 includes a tubular fuel cell 10, which is illustrated in greater detail in FIG. 2. Although a tubular fuel cell is illustrated, it is well-known in the art that fuel cells may be formed in many different shapes and configurations. The fuel cell 10 illustrated in FIG. 2 includes an air electrode 12 on the inner circumference of the fuel cell 10 and a fuel electrode 14 on the outer circumference of the fuel cell 10. The air electrode 12 and the fuel electrode 14 are separated by an electrolyte 16.
Electricity is generated in the fuel cell 10 in a catalytic reaction when input air passes over the air electrode 12 and input fuel passes over the fuel electrode 14. The catalytic reaction produces ions in the air electrode 12 and the fuel electrode 14. These ions move through the electrolyte 16 toward the oppositely charged electrode, producing electricity and certain by-products such as water and CO2. Once produced, the electricity may be transferred to an external circuit to serve an electric load.
The specific chemical reactions that occur in a fuel cell depend on the input fuel and the electrolyte utilized by the fuel cell. The most common types of fuel cells are phosphoric-acid fuel cells, molten-carbonate fuel cells, proton-exchange-membrane fuel cells, and solid-oxide fuel cells. In a solid-oxide fuel cell, for example, which utilizes a ceramic-solid-phase electrolyte, hydrogen or carbon monoxide in the fuel reacts with oxide ions (O=) from the electrolyte to produce water or CO2 and also to deposit electrons in the anode:
Anode ReactionH2+O=→H2O+2e−CO+O=→CO2+2e−CH4+4O=→2H2O+CO2+8e−Cathode ReactionO2+4e−→2O=
The chemical reactions in a fuel cell, and the performance characteristics of a fuel cell, are very sensitive to changes in the operating temperature of the fuel cell. To achieve adequate ionic conductivity in a solid-oxide fuel cell, for example, the fuel cell must operate between about 800° C. and 1200° C., and preferably at about 1000° C. If the operating temperature of the fuel cell drops by 10 percent, the fuel cell's performance may drop by 12 percent. Thus, it is very important to maintain a stable operating temperature in a fuel cell power generator. A problem exists, however, in that conventional fuel-cell-temperature-control systems permit significant overshoots and/or under-swings in the operating temperature of a fuel cell.
FIG. 3 illustrates a conventional-fuel-cell-temperature-control system 20. The temperature-control system 20 includes a fuel cell 22, an air mover 23, a heat exchanger 24, a bypass valve 25, a heater 26, and a controller 27. The conventional temperature-control system 20 operates to stabilize the operating temperature of the fuel cell 22 by varying the amount of heat that is added to the inlet air by the heat exchanger 24 and the heater 26.
As changing conditions in and around the fuel cell 22 cause the actual operating temperature of the fuel cell 22 to deviate from the desired temperature set-point, the controller 27 attempts to bring the operating temperature back to the desired temperature set-point by varying the amount of heat added to the inlet-air stream by the heat exchanger 24 and the heater 26. However, because the thermal diffusivity (i.e., the ratio of thermal conductivity to mass density) of the fuel cell is low, the fuel cell system has a high thermal inertia. This makes it difficult for the conventional-temperature-control system to accurately maintain the operating temperature of the fuel cell by merely adjusting the amount of heat added to the inlet air stream. As a result, the conventional-temperature-control system 20 often permits significant overshoots and/or under-swings in the operating temperature of the fuel cell 22.